The discovery of a B⁹ isochromosome was reported in 1970 (Chromosoma 30:356). Nondisjunction of the chromosome occurred at variable rates, and an average value of 20% was found. Since the isochromosome carried out nondisjunction at significant levels, it was assumed that none of the genes required for nondisjunction was missing. Subsequently, instability of the B⁹ isochromosome became apparent (Chromosoma 42:127). Twenty-two endosperm mosaics were found in a single ear of a c c x 9^B 9^B iso-B⁹ C C cross. Cytological examination of plants from the twenty-two kernels showed that telocentric formation was associated with mosaicism. Elaine Brannen (unpublished) traced telocentric formation to the second pollen mitosis. The iso-chromosome is now considered to lack a factor, probably on the B short arm, required for efficient nondisjunction. Absence of the factor often leads to centromeric damage at the second pollen mitosis and telocentric production. However, the isochromosome also frequently undergoes nondisjunction, with no centromeric damage.

In this article, properties of the isochromosome in 9 9^B iso-B⁹ and 9^B 9^B iso-B⁹ plants are given. The plants were derived by self pollination of a 9 9^B iso-B⁹ individual. Rates of nondisjunction and endosperm mosaicism were determined for 9^B iso-B⁹ pollen. (In 9 wx, 9^B Wx, iso-B⁹ plants, Wx progeny were selected, since the locus effectively mark 9^B iso-B⁹ pollen). In addition, crosses of 9 9^B iso-B⁹ plants were examined for competition between 9 wx and 9^B Wx, iso-B⁹ pollen types by classifying Wx vs wx. Data from the crosses are given in Tables 7 and 8.

The four 9^B 9^B iso-B⁹ plants tested gave an average rate of 14% bz Wx kernels. Nondisjunction, assuming preferential fertilization, was, therefore, approximately 14% x 1.5 = 21%. The eight plants with the 9 9^B iso-B⁹ chromosome combination showed the same average rate of nondisjunction. Endosperm instability in these crosses most frequently produced multiple recessive sectors, suggestive of bridge-breakage-fusion cycles. The average frequency of multiples for 9^B 9^B iso-B⁹ plants was 2.3%. In 9 9^B iso-B⁹ crosses, multiples were found in 4.3% of cases. In general, endosperm sectoring was more frequent for 9 9^B iso-B⁹ than 9^B 9^B iso-B⁹ plants. However, genetic background rather than 9^B chromosome content may be responsible for the difference. Transmission of the 9^B iso-B⁹ chromosome combination (% Wx) through pollen of the 9 9^B iso-B⁹ plants ranged from 16% to 32% and averaged 24%. This is about one-half the rate found for standard 9^B B⁹ pollen in a similar situation. Reduced transmission of isochromosome-containing pollen in competition with normal (9) pollen reflects the harmful effects of gene duplication.

Table 7.

Table 8.

Since the B⁹ isochromosome shows frequent instability when crossed as male parent, cases of nondisjunction detected genetically as bz Wx kernels are suspect. The bz phenotype can result from nondisjunction with two isochromosomes in the embryo. However, 0-0 or 0-1 disjunction could also produce bz kernels. Consequently, twenty-four bz Wx kernels from crosses of plants 2011A and 2011M were germinated for examination of root tip mitoses. Among fourteen progeny of 2011A all had two isochromosome B⁹'s. Nine plants from 2011M had two isochromosomes, and one plant had a single isochromosome. Although a single case of 0-1 disjunction was found, the predominant event was conventional nondisjunction.

Wayne R. Carlson

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